Hydrological characterisation of Ria de Aveiro, Portugal, in early summer

The hydrological features of Ria de Aveiro, a coastal lagoon on the northwe st Atlantic coast of Portugal, were investigated in two sampling surveys ca rried out between 3/6 and 24/6/97 and between 29/6 and 7/7/97, respectively There was a significant freshwater inflow into the lagoon during the first survey, especially in the first days, due to the recent rainfalls. Records concerning water level, salinity, temperature and current velocity were pe rformed at several stations located along the four main channels of the lag oon. The type of tide at the mouth was determined and was observed that ast ronomical tide is the main forcing agent driving water circulation in Ria d e Aveiro. The tide at the mouth is semidiurnal and the tidal wave propagati on in the lagoon has the characteristics of a damped progressive wave. Typi cal estuarine longitudinal salinity and temperature gradients connected wit h the distance to the mouth were identified, whereas vertical and transvers e gradients were found unimportant, except in the frontal zone between ocea nic and fresh water masses. According to the results, even though Ria de Av eiro should be considered as vertically homogeneous, some channels may reve al characteristics of a partially mixed estuary, depending on the freshwate r input. (C) 1999 Ifremer / CNRS / IRD / Editions scientifiques et medicale s Elsevier SAS.